The current phase of this project will involve investigating the hemodynamic responses of postmenopausal women to maximal tolerable passive heat stress. Specifically, the separate effects of hormone replacement therapy and hydration on these responses will be studied. Two experiments will comprise this study which will be conducted over a one year period. In the first experiment, 8 postmenopausal women will be recruited into each of three groups -- one group of women taking estrogen replacement therapy, one group of women taking combined estrogen and progesterone replacement therapy and one group of women not taking any hormone replacement therapy. The goal of this phase is to determine the effects of hormone replacement therapy on cardiac output and its distribution during maximal tolerable passive heat stress. In the second experiment, 8 women not taking any hormone replacement therapy will undergo maximal tolerable passive heat stress on three separate occasions under three separate hydration conditions -- after eight hours of fluid deprivation, after hydration with 7.5 ml/kg body weight of water, and after hydration with 7.5 ml/kg body weight of a commercially available sports drink. The purpose of the second experiment is to determine the effects of hydration on the distribution of cardiac output.